Method of and apparatus for carrying out measurements on open and closed fractures in a hard rock formation pierced by a borehole

Abstract

 The invention refers to a method of and an apparatus for carrying out measurements on open and closed fractures in a hard rock formation pierced by a borehole. According to the method a main current and a controlled bucking current are fed into the hard rock formation, dimensionless ratios are determined on the basis of the measured current and potential values and the open and closed fractures are differentiated on the basis of the values received. The proposed apparatus comprises at least one measuring pad (6) arranged on an arm (7) of a downhole measuring tool (1) suspended in the borehole, the measuring pad (6) made of insulating material and comprising an electrode system, and a current and potential measurement system connected with data processing means. The electrode system includes a point-like central electrode (14), an outer measuring electrode (13) surrounded by a first feeding electrode (9) and a second feeding electrode (10) limiting a first and a second monitoring electrodes (11, 12) for regulating bucking current supplied by the second feeding electrode (10), wherein the first and second monitoring electrodes (11, 12) are connected to respective inputs of a controlled current generator (16) for supplying the bucking current to the second feeding electrode and the first feeding electrode is connected to a main current generator (15).

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of and an apparatus for carrying out measurements on open and closed fractures in a hard rock formation pierced by a borehole. The method and the apparatus proposed are based on generating microelectric field penetrating the wall of the borehole. According to the invention it is possible to determine the network of the fractures present in a hard rock formation, especially to differentiate the open fractures constituting passageways connected into a system of communicating vessels showing remarkable hydraulic conducti­vity (permittivity) from the closed fractures being not permeable from outside of the borehole.

[0002] The apparatus and the method of the invention can be applied especially in the petrologic and hydrogeologic investigations and in displaying, prospecting the sources of useful minerals.

[0003] The determination of the network of the fractures penetrating a hard rock formation pierced by a borehole is a very important object of the geophysical investiga­tions carried out through a borehole. In the hydrogeology, petrology and in the process of displaying the useful minerals the data received in this way are evaluated and processed under geophysical, geologic and mineralogic aspects. The importance of the measurements of such kind follows also from the well known fact that the fractures present in a hard rock formation may form the way whereby water breaks in and causes by water flooding high damages to the mines.

[0004] The background art is consisted of different solu­tions to the problem mentioned above; the most developed of them may be identified with the methods shown in the Letters Patent US-A 4 468 623 and in the European early publication EP-A3-0 287 320. The essence of the methods cited above lies in application of a measuring downhole tool comprising pads made of electrically insulating material and a system of metallic electrodes arranged within the pad. During the measurements the downhole tool is lowered and pressed to the wall of the borehole in different places. The electrodes generate a micro­electric field in theire narrow environment. The micro­electric field penetrates the rocks adjacent to the in­sulating pad and is distorted by them. The parameters of the microelectric field are measured and the data obtained thereby analyzed. The measured parameters are the electric current intensities and voltages and by processing them it is possible to determined whether fractures are present in the region investigated.

[0005] In the references cited a method applicable to the measurements of sharp local inhomogeneities on the wall of a borehole in layers pierced by this borehole is proposed, whereing an electric field is generated by microelectrodes in a conductive way on the wall of the borehole. The microelectrods are shaded from the drilling mud filling the borehole by insulating pads excluding direct electric contact to the conductive liq­uid present ind the layers pierced by the borehole. In the next step the current are determined at two or more localizations in the borehole by carrying out local meas­urements. The current intensities are coupled in a parallel way with one another in a current field generated perpen­dicularly to the wall of the borehole in the rock shaded from the drilling mud. The local inhomogeneities are de­termined on the basis of comparing the parallel current intensities: for each measurement the dimensionless ratio of the current intensities is computed and the maximum values of the dimensionless ratios are applied for dis­playing inhomogeneities on a major surface area divided into smaller regions.

[0006] The mentioned methods and apparata ensure highly effective measurements of the inhomogeneities on the wall of a borehole. However, they are characterized by two main disadvantages of generic character:

i. The investigations penetrate the rocks with shallow depth, i.e. in the practice it is impossible to differentiate the open fractures forming parts of a communicating vessel system showing remarkable hydraulic conductivity (permittivity) from the closed fractures produced mainly by the boring operation due to altering the stress distribution system in the interior of the rock. This impossibility follows from the fact that the systems of both fractures are deeper than the length of penetration of the microelectric field into the rock formation.

ii. The background art is based on the obviously inaccurate assumption that the fractures constitute gene­rally straight line formations in the wall of the borehole. Hence, during the data processing the fractures are sup­posed to traverse the rock along straight lines. This assumption results in erronous determination of the trend directions of the fractures - the fractures traversing the rocks along broken and curved lines can not exactly be determined, however, the majority of the fractures is not straight.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to elaborating a method of and an apparatus for carrying out measurements on the uneven wall of the borehole in order to differentiate the mentioned kinds of fractures and to determine the geometric distribution of the fractures within the hard rock formation. Thus, the invention proposes a method and an apparatus for obtaining highly reliable data con­stituting basis of determination the fracture structure in a wide environment of the borehole.

[0008] Hence, according to the first object of the present invention a controlled and focused microelectric field should be generated in a space limited from one side by an inner protected surface of an insulating pad pressed to the wall of the borehole. In this space a circular form, generally ring shaped measuring electrode M is plac­ed. To the protected surface remarkable current transport can be observed from the microelectric field only in the case, when on the wall of the borehole there is a fracture transversing the area investigated by the pad, the fracture being present oppositely to the protected surface. The measuring system generates in this case a signal corresponding to a dimensionless parameter U_MK/U_K representing a ratio of current transport values. The first of them, U_MK is proportional to the current trans­port from the ring shaped measuring electrode M to a central measuring point K lying within the protected surface. The second member of the ratio, U_K is proportional to the current transport in direction perpendicular to the surface of the pad, the second member being measured in the central measuring point K with respect to a far reference electrode N. The dimensionless ratio U_MK/U_K has a very low, nearly zero value if the wall of the borehole is generally free of fractures and shows remark­ ably higher value if a fracture transverses the wall of the borehole oppositely to the protected surface of the insulating pad.

[0009] The dimensionless ratio U_MK/U_K is therefore an indicator of the presence of a facture, however in itself it doesn't mean a reliable basis for differentiating the fractures according their hydraulic conductivity, because of shallow depth of the invention. Hence, this ratio is only a piece of information on the presence of the fracture.

[0010] The recognition is that a further dimensionless ratio I₁/I₂ of two current fluxes is characteristic for the kind of the fracture, i.e. this ratio can be the basis of differentiating the open fractures from the closed ones.

[0011] The current flux I₁ is the main measuring current of the microelectric field and it is fed into the space filled with the rock from a ring shaped metallic electrode A₁ arranged around the central measuring point K, adjacent­ly to the outer environment of the inner protected surface area of the insulating pad. The second current flux signed by I₂ is a bucking current fed into the rock environment by another ring shaped electrode A₂ having radius greater than that of electrode A₁. The second current flux I₂ is sharply focused by arranging two further ring shaped electrodes S₁ and S₂ between the mentioned ring shaped metallic electrodes A₁ and A₂. The magnitude of the current flux I₂ is regulated in an automatic system with respect to the current intensity I₁ in order to ensure by the cur­rent transport between the regulating ring shaped electrodes S₁ and S₂ a potential difference ΔU with value near or practically equal to zero.

[0012] This method of the automatic regulation results in high value of the inner current flux I₁ with respect to the second current flux when in the hard rock formation an open, hydraulically conductive fracture is present at the inner protected surface of the downhole investigation pad, the fracture facing the ring shaped current electrodes A₁ and A₂. Because of the focusing influence exerted by the second current flux assigned to the bucking current I₂ the first current flux I₁ assigned to the main current de­creases it there is no open fracture in the measurement area. The lack of an open fracture with remarkable hydrau­lic conductivity means that there is no permeable and conducting medium through the hard rock formation wherein the electric current can flow and the current escape through the film of the milling mud between the measurement pad and the hard wall of the borehole is prevented by the focusing current transport I₂.

[0013] According to the concusions made above it can be stated that the dimensionless ratio I₁/I₂ has a very low, nearly zero value in lack of the open fractures and it shows a remarkable increase in value if the arrangement faces an open fracture, due to the increased permittivity and conductivity of the medium filling out the inner space of the open fractures. Thus, the dimensionless ratio U_MK/U_K indicates whether a closed or open fracture is pre­sent in the interior of the field limited by the ring shaped electrodes, and the dimensionless ratio I₁/I₂ gives the basis of differentiating the open fractures from the closed ones, giving relatively high values in the case of open fractures, i.e. fractures formint part of a communicating vessel system with remarkable hydraulic conductivity (permittivity).

[0014] The invention proposed further to introduce the dimensionless normalized product

computed from the mentioned dimensionless indicating ratios on the basis of known mathematical constants A and B reflecting the given geophysical circumstances, geologic conditions. This product is especially capable of diffe­rentiating the open fractures from the closed ones be­cause it attenuates the indication whenever the fracture is a closed formation and amplifies, increases the indica­tion when answering to the open fractures. The increase itself depends on the hydraulic conductivity of the frac­ture identified by the pad.

[0015] The second object of the present invention is to provide a method for determining the network of the open fractures showing a given level of the hydraulic conducti­vity and being identified according to the first object depicted above. The method as proposed is based on further important steps following the determination of the mentioned dimensionless ratios. According to the method the inner protected surface of the insulating pad is limited by a measuring ring M divided into more segments. Generally, the number of the segments lies in the range from 4 to 24, not compulsory. For practical purposes the numer of 12 seems to be fully acceptable, it can ensure the required accura­cy. For each segment a further dimensionless ratio U_MK,i/U_K is determined, wherein U_MK,i means the current transport between the i-th segment of the measuring ring M and is expressed in voltage terms. The current transport U_K refers to the absolute potential value of the central measuring point K related to a far reference electrode. This dimen­sionless ratio can be regarded as an elementary fracture indication having value obviously depending on the relative arrangement of the fracture line in the moment of the meas­urement with respect to the straight line connecting the i-th measuring segment of the ring M with the central meas­uring point K. According to the method presented here the elementary fracture indication is determined for each segment of the segmented measuring ring (the number of the segments is n), i.e. the elementary fracture indications U_MK.1/U_K, U_MK,2/U_K, ..., U_MK,n/U_K form the basis of deter­mination of the geometric orientation of the fracture facing the insulating pad.

[0016] It is clear that in this method there is no general preconception that the open fractures transverse the inner protected surface of the pad along a straight line. The geometric arrangement of the fracture can be determined according to reality: a good approximation of the broken or curved line is obtained.

[0017] In order to determine the geometric arrangement of the fracture as it stands the elementary fracture indi­cations U_MK,i/U_K are tested and thereamong the two with local maxima (e.g. U_MK,j/U_K and U_MK,m/U_K, belonging to the j-th and m-th segments) are selected. The straight lines connecting the j-th and m-th segments with the cent­ral measuring point K generally lie not on a common straight line, i.e. not along the same diagonal crossing the central measuring point K. A straight line fracture can be deter­mine therefore on this basis.

[0018] According to this second object of the invention the measurement of the real geometric orientation of the fractures in their network is ensured by carrying out a series of determinations. The momentary position of the pad of the downhole measuring tool is changed during the measurements, therefore the series of the determina­tions give a set of data. Because of the known speed of lowering the downhole investigation tool the speed of movement of the central measuring point K is given and on this basis, by processing the data obtained during the well logging process the geometry of the fractures with respect to the straight line of lowering can be com­puted.

[0019] The third main object of the invention is to apply the principle of plurality, i.e. to carry out measurements in more points simultaneously. This means, the orientation of the open fractures with remarkable hydraulic conducti­vity (permittivity) is determined in more points in the same time. The number of the measuring points vary between 2 and 8, generally 4 points are applied. Obviously, the number of the measuring points can be as high as necessary. On the basis of the simultaneous measurements a continuous picture of the fractures in the hard rock formation can be obtained, in contrary to the measurements carried out in separate points of the cross section of the borehole.

[0020] In given circumstances the measurements carried out simultanously can interfere one with another. This is avoided by the method of the invention by selecting diffe­rent frequency values for ensuring the independency of the separate measurements. This means, different frequency values signed by f₁, f₂, f₃, f₄ (if 4 pads are applied) are selected for generating the microelectric fields in the pads. Generally, the frequency values not exceed a limit 1 kHz.

[0021] The fourth object of the present invention is to make use of electrodes divided into more segments instead of the ring shaped main current electrode A₁ forming a continuous ring. The potential of the microelectric field generated by the segments should be constant. The equipo­tential system of the segments renders it possible to avoid current flow from the electrode segments A₁ to the inner part covered by a surface limited by the electrode segments A₁ if the wall of the borehole is free from frac­ tures. The equal potential of the segments is ensured by connecting them in series through very low value ohmic re­sistors. The resistor connected in series with the given segment can be applied also for the measurement of that part of the basis current which is fed into the rocks over the given segment. The partial current fed by the given i-th segment signed by I_1,i (wherein i means an integer from l to n) is applied for determining a dimensionless ratio by dividing it by the current flux I₂ defined above. The dimensionless ratio I_1,i/I₂ is also a fracture indica­tion giving a piece of selective information of place and direction of the fracture, similarly to the partial fracture indications U_MK,i/U_K defined earlier in the case of the measuring electrode M divided into more segments. By forming a segmented ring shaped electrode A₁ it is possible to en­hance the directional sensitivity of the measurements and a further very advantageous feature is received: the mud layer or mudcake between the pad of the downgole investi­gation tool and the compact waof the borehole can be detect­ed and its thickness can be determined.

[0022] This is carried out in the following way:
The first step is to determine for the segmentes ring shaped main current electrode A₁ the mean (average) value I_1,i of the partial currents defined above:

The mean value I_1,i is subtracted from the partial currents I_1,i and the difference thus obtained is divided by the current flux I₂. The dimensionless normalized values re­ceived for each of the segments, i.e. the dimensionless ratios

are then divided into two groups: the first of the two groups belong the highest values and at most the first four of them, the value exceeding a predetermined thresh­old ε; the relatively high values give evidence of frac­tures transversing the lines of corresponding segments. Hence, the serial numbers (e.g. : j, m, p, v) of these seg­ments ensure the possibility of determining the features of the fractures with respect to their directions and arrangements. If all mentioned ratios have values below the mentioned threshold ε, it means that there is no frac­ture in the region actually detected by the pad, but there is a mud layer between the pad and the fracture-free wall of the borehole. In this case the mentioned dimensionless normalized ratios are to be proved whether they fall below another predetermined threshold ω; if they do, it means the mud layer is obviously a mudcake lying on the wall of the borehole, and if not, the mud layer consists of real mud filling the space between the pade and the wall of the borehole, the last having a not smooth but fragmentarized (rugose) surface. Hence, according to the invention it is possible to differentiate the mudcake fromthe rugosity of the wall of the borehole, further to determine the thick­ness either for the mudcake or for the rugosity (both can be signed by t_m), because the higher the value of the di­mensionless ratio I_1,i/I₂ the thicker the mud layer (or the rugosity). The thickness t_m can be determined with high ac­curacy on the basis of a functional dependency ot the thickness t_m on the dimensionless ratio I_1,i/I₂. This means, if the ring shaped main current electrode A₁ is di­vided into segments it is possible to obtain, a fracture indication value H_e of very rich informative content which value differs from the indication value H defined above because of applying the sum of the partial current I_1,i exceeding the first mentioned threshold ε, the sum re­placing the full main current flux I₁:

This indicator es equal to zero whenever there isn't any partial current exceeding in value the mentioned thresh­old ε. The zero level of the indicator H_e defined above is an evidence for lack of the fractures. The level of the indicator H_e differing from zero shows the magnitude of the hydraulic conductivity (permittivity) of the frac­ture; the accuracy of this determination is higher than that of the determination based on the previously defined indicator H.

[0023] The mathematical constants A and B and the thresh­olds ε and ω have values depending on the given circum­ stances, on the geophysic and geologic conditions of the measurements.

[0024] The fifth object of the present invention is to pro­vide an improved electronic arrangement and system for reg­ulating the second current flux I₂ (bucking current). The regulation is achieved by ring shaped electrodes A₂ and A₃ surrounding the ring shaped main current electrode A₁ and replacing the electrodes S₁ and S₂ for detecting voltages. The msin current electrode A₁ can be divided into segments if required. In this embodiment the electrodes K and M meas­uring predetermined kinds of voltage lack and around the ring shaped main current electrode a two part electrode system is arranged, consisting of two ring shaped electrodes A₂ and A₃. The last form conductive rings connected one with another and through respective resistors of very low ohmic values with respective output(s) of a generator for producing the main current I₁ (or the partial currents I_1,i of the segments). Hence, the potential of the ring shaped electrodes A₁, A₂ and A₃ is always the same. Between the rings of the electrodes A₂ and A₁, further A₃ and A₂ there are respective potential troughs (regions charac­terized by low potential values) in the space between the surface of the insulating pad of the downhole tool and the wall of the borehole. The mentioned two potential troughs prevent current leakage from the main current flux I₁ in absence of fractures, when the open space between the pad and the wall of the borehole is filled with the drilling mud of relatively high electric conductivity. These two potential troughs are capable of creating condi­tions which are the same as in the case of applying the electrodes S₁ and S₂ for detecting potential, when the ring shaped electrode A₂ is applied for feeding in a bucking current I₂ in order to ensure zero potential difference between the electrodes; the solution proposed here is not so complicated.

[0025] The _sixth object of the present invention is to apply an arrangement for selecting the peak value of the dimensionless fracture indicator U_MK/U_K. According to the invention this can be achieved by replacing the small electrode forming the central measuring point K by a ring shaped electrode of relatively great diameter. In this case the peak value of the dimensionless fracture indicator U_MK/U_K can be longer observed: the high measurement level exists as long as the ring shaped electrode K faces the fracture during the movement of the downhole tool and this time is determined by the diameter of the ring of the measuring electrode M. Due to the longer time it is possible to carry out a high accuracy measurement of the peak value assigned to the dimensionless ratio U_MK/U_K whereby the accuracy of determining the hydraulic conductivity (permit­tivity) increases, too. The increase follows from the improved determination of the fracture indicators H and H_e.

[0026] The _seventh object of the present invention is to ensure further enrichment of the informative content of the data obtained by the measurement. According to a modi­ fied embodiment of the proposed method in the fracture in­dicators H and H_e the exponent B is selected to have a value reflecting the given conditions (this exponent is signed further by Y) and the coefficient A is given the value 1.0. Further, the fracture indicator U_MK/U_K is raised to an X-th power. The exponents Y and X can be determined by the means of the regression analysis. In this way modi­fied hydraulic fracture indicators M′ and M

can be obtained which are defined as follows:



The values of the modified fracture indicators M′ and M

are related to the earlier introduced fracture indicators H and H_e with differences listed up above. The exponents X and Y (similarly to the exponent B and the constant A) should be determined taking into account the specific local features of the oil/gas and water bearing reservoirs to be investigated. The methods of detemining are known from the regression analysis (i.e. from the mathematical statistics).

[0027] The eighth object of the present invention to apply once more the principle of the plurality, i.e. to carry out simultaneous measurements oblong the circumference of a cross section of the borehole. The data obtained from the simultaneous measurements carried out in 2 to 8, pre­ferably 4 points renders it possible to determine a con­tinuous picture of the features of the borehole and its environment. Obviously, the picture prepared continuously has a very rich informative content with respect to that of based on single measurements.

[0028] Hence, the present invention as depicted above in accordance with different objects offers a solution for differentiating the open, hydraulically conductive (i.e. forming parts of a system of communicating vessels) fractures from the closed fractures showing low or neg­ligable hydraulic conductivity.

[0029] The method and apparatus of the invention give an adequate basis for determining the directions of travers­ing the rocks by the fractures.

[0030] The basic idea of the invention is to combine two recognition, i.e. the measurement of a dimensionless current ratio characterizing the open fractures, and the measurement in a central shaded point of an electrode system.

[0031] The main advantage of the invention is that the directions of the fractures can be determined, too. This is ensured by altering the place of detecting the potential values and/or introducing currents, by segmented embodi­ments of the electrodes.

[0032] A further unexpected advantage is that the combina­ tion gives adequate data for differentiating the hydraulic connections present on the rugose (uneven) wall of the borehole from the hydraulic connection system really present in the fracturized hard rock formation.

[0033] The method and the apparatus according to the in­vention give therefore a reliable basis for determining the most important geophysic parameters of a hard rock formation pierced by a borehole. The measured data of the fractures present in the hard rock formation give the possibility of determining whether the fractures measured are natural formations or resulted in the process of lower­ing the borehole.

DESCRIPTION OF THE DRAWINGS

[0034] The invention will be further explained in more detail with reference to preferred embodiments and reali­sations shown in the accompanying drawings wherein

Fig. 1 is a lateral view of the downhole too lowered into the borehole with pads pressed to the wall of the borehole, with cross section of the hard rock for­mation,

Fig. 2 shows a schematic diagram of a first embodiment of the apparatus proposed by the invention with a continuous measuring ring M,

Fig. 3 is a front view of the measuring pad of the down­hole tool having block diagram according to the Fig. 2,

Fig. 3A is a cross section taken along a line A-A of the measuring pad of the Fig. 3,

Fig. 4 is a schematic circuit diagram of a second embodi­ment of the proposed apparatus wherein a measuring ring M divided into n = 12 segments forming elemen­tary measuring points is applied,

Fig. 5 shows a front view of a measuring pad of the down­hole tool with electrodes arranged according to the Fig. 4,

Fig. 5A is a cross section taken along a line A-A of the measuring pad shown in the Fig. 5,

Fig. 6 represents a horizontal cross section VI-VI of a four pad embodiment of the downhole investigation tool shown in Fig. 1, the four pads arranged in a system for carrying out simultaneous measurements,

Fig. 7 is a schematic circuit diagram of a third embodiment of the apparatus proposed by the invention, wherein the apparatus is equipped with a measuring elect­rode A₁ divided into n = 12 segments for feeding in partial current fluxes, and

Fig. 8 is a schematic arrangement of the inputs and out­puts of an enlarged aritmetic unit represented in the schematic circuit diagram of Fig. 7.

[0035] As it is shown in Fig. 1, a downhole measuring tool 1 for investigating fractures is suspended on a well logging cable 3 which is connected to a head unit 4 of the downhole measuring tool 1. The suspension is realised in a borehole filled with drilling mud 2 and lowered in a hard rock formation 5 to be investigated. The well logging cable 3 connects the downhole measuring tool 1 with a surface operating unit (not shown in the drawings), comprising means for data processing. The downhole measuring tool is built up with appropriate arms 7 bearing respective measuring pads 6/1, 6/2, 6/3 (signed generally by 6 in Figures 3 and 5) made of insulating material and equipped with an electrode system for carrying out the measurements and for­warding the necessary currents. The number of the measuring pads 6 is not limited to three as shown in Fig. 1, it can be, if required, higher or lower, depending on the conditions of the measurement. The pads 6/1, 6/2 and 6/3 are pressed on the arms 7 to a wall region 8 of the borehole.

[0036] The surface operating unit includes most of the electronic elements listed up in the following. Of course, only the equipment arranged on the pads 6 constitute those parts of the apparatus as proposed which obviously must be arranged in the downhole measuring tool 1, the further elements and parts may be arranged either in the downhole measuring tool or in the surface operating unit. However, the general praxis is to apply the units needed for data processing in the surface operating unit.

[0037] Fig. 2 represents a schematic circuit diagram of a first embodiment of the apparatus proposed by the invention, wherein an electrode system consisting of ring shaped elements is arranged on a measuring pad 6. The downhole measuring tool 1 (Fig. 1) is generally equipped with a higher number of pads built up with circuit diagram accord­ing to Fig. 2.

[0038] The measuring pad 6 bears a central electrode 14 for voltage measurements. The central electrode 14 is con­stituted by a spot-like metallic element, however, it may be made also in form a ring having small diameter. The meas­uring pads 6 is built up with a carrier body 36 (Fig. 3 and 5) made of electrically insulating material. The mentioned system of the electrodes is arranged on one side of the carrier body 36 and oppositely to this side the carrier body 36 with the arm 7, whereby a direct contact between the electrodes and the wall region 8 is realized. The arm 7 presses the carrier body 36 to the wall region 8 and thereby the electric contact between the drilling mud 2 and the sys­tem of electrodes is excluded as shown in Figures 3A and 5A, i.e. from the side of the arm 7 (not shown in Figures 3A and 5A) the drilling mud 2 can not come into contact with the system of electrodes contacted with the wall re­gion 8 of the borehole.

[0039] The central electrode 14 is surrounded by an outer measuring electrode 13, followed - from inside of the meas­uring pad 6 to its circumference - by a first feeding electrode 9, a first and a second detecting electrodes 11 and 12 and a second feeding electrode 10. The first and second detecting electrodes 11 and 12 are applied for pro­viding observation of the potential conditions as it will be described later.

[0040] The first feeding electrode 9 is connected over a first output measuring resistor 18 to a first output of a main current generator 15 having a further output connected to a far return feeding electrode 14. The first output measuring electrode 18 is inserted also between two respec­tive inputs of an ammeter 19 having an output coupled with an input of a first arithmetic data processing means 21 equipped with output means 21/1, 21/2, 21/3 and 21/4 for forwarding respective signals according to the measured pa­rameters.

[0041] The second feeding electrode 10 is connected over a second output measuring resistor 17 to a first output of a controlled current generator 16 coupled through its second output to the far return feeding electrode 14. The controlled current generator 16 for generating bucking current is con­nected through its inputs with the first and second detect­ing electrodes 11 and 12. The first output measuring resistor 18 is active when measuring the main current supplied to the first feeding electrode 9 and the second output measuring resistor 17 checks the bucking current fed from the controlled current generator 16 to the second feeding electrode 10 under influence of the input state determined by the first and sec­ond detecting electrodes 11 and 12.

[0042] The second output measuring resistor 17 is connecting between two inputs of an auxiliary ammeter for measuring the bucking current produced by the controlled current generator 16, and the output of the auxiliary ammeter 20 is coupled with an input of the first arithmetic data processing means 21.

[0043] A unit 22 for measuring voltage difference is connect­ed through its inputs with the outer measuring electrode 13 and the central electrode 14, which is in the case of the circuit diagram shown in Fig. 2 a point-like metallic ele­ment. One input of the unit 22 is connected to a first input of a voltmeter 23 connected by its second input to a far re­ference electrode 25 supplying reference voltage for carry­ing out measurements of absolute value of the voltage. The output of the voltmeter 23 and the unit 22 as well are con­nected to respective inputs of the first arithmetic data processing means 21.

[0044] The outputs 21/1, 21/2, 21/3, 21/4 of the first arith­metic data processing means 21 forward respective measurement data to further data processing means.

[0045] The carrier element 36 of the pad 6 with circuit diagram according to Fig. 2 is shown in front view (from the side pressed to the wall region 8 of the borehole) in Fig. 3 and in cross-section A-A in Fig. 3A. The curved surface of the measuring pad 6 is pressed against the investigated wall region 8 in the hard rock formation 8 in a way that there is no electric contact between the electrode system of the measuring pad 6 and the drilling mud 2. The position of the carrier body 36 of the measuring pad 6 is secured by the arm 7 of the downhole measuring tool 1 (not shown in these Figures). The measuring pad 6 bears the point-like central electrode 14 and the surrounding further ring shaped electrodes, i.e. the outer measuring electrode 13, the first feeding electrode 9, the first and second detecting electrodes 11 and 12 and the second feeding electrode 10. The elements for fitting the measuring pad 6 are not shown in Figures 3 and 3A.

[0046] A further advantageous embodiment of the apparatus proposed by the invention is represented by a circuit dia­gram shown in Fig. 3. This embodiment is capable of carry­ing out measurements of the system of open fractures termi­nating at the investigating wall region 8. According to the circuit diagram shown in Fig. 4 the ring shaped first feed­ing electrode 9 is connected by the first output measuring resistor 18 to the first output of the main current gener­ator 15, the second output of which is coupled with the far return feeding electrode 24. The ring shaped first and sec­ond detecting electrodes 11 and 12 are coupled with respec­tive inputs of the controlled current generator 16 supplying bucking current to the second feeding electrode 10 through the second output measuring resistor 17 under influence of the signals received through respective inputs from the first and second detecting electrodes 11 and 12. The second output of the controlled current generator 16 is coupled also with the far return feeding electrode. The first output measuring resistor 18 is connected with the ammeter 19 for measuring the main current supplied by the main current ge­nerator 15 and the second output measuring resistor 17 is arranged at the input of the auxiliary ammeter 20 intended to determine the bucking current.

[0047] The difference of this embodiment to that shown in Fig. 2 is that the outer measuring electrode 13 is divided into more segments, some of them signed by 13/1, 13/j, 13/m and 13/12. The number of segments of the outer measur­ing electrode 13 is not specific, it depends on the given circumstances. An advantageous number is e.g. twelve as depicted in Fig. 4. The segments of the outer measuring electrode 13 constitute a segmented ring arranged concent­rically to the central electrode 14. Each of the segments is connected to respective units for measuring voltage difference, whereunder only four signed by 22/1, 22/j, 22/m, 22/12 are represented in Fig. 4. A common input of the units for measuring voltage difference is connected with the central electrode 14, and thereby the central electrode 14 is linked with the voltmeter 23 having an input driven from the far reference electrode 25 creating the possibility of determining the absolute value of the voltage. The Fig. 4 shows only the segments signed by 13/1, 13/j, 13/m, 13/12 connected to the inputs of the units 22/1, 22/j, 22/m, 22/12 for measuring voltage diffe­rence, however, obviously all segments of the outer meas­uring electrode 13 are assigned to respective units for measuring voltage difference, if necessary.

[0048] The outputs of the ammeter 19 and auxiliary ammeter 20, the units 22/1, 22/j, 22/m, 22/12 and the voltmeter 23 are coupled with respective inputs of a second arithmetic data processing unit 26 having outputs 26/1, 26/2, 26/3, 26/4, 26/5, 26/6, 26/7 for forwarding signals according to the measured data.

[0049] The measuring pad 6 of the embodiment represented by the circuit diagram of Fig. 4 is shown front view in Fig. 5 and in cross-section A-A in Fig. 5A. In this case also the system of the electrodes is arranged on the carrier body 36, on its side facing the wall region 8 to be inves­tigated in the borehole. The inner curved surface of the car­rier body 36 connected with the arm 7 (not shown in Figures 5 and 5A) faces the drilling mud 2. The outer curved surface is pressed to the wall region 8 in a way excluding the di­rect electric contact between the drilling mud 2 and the electrode system. The measuring pad 6 is equipped with the point-like central electrode 14 and the further surrounding ring shaped electrodes, i.e. the out measuring electrode 13 divided into segments 13/1, 13/i, 13/j, 13/m and 13/12, the first feeding electrode 9, the first and second detecting electrodes 11 and 12 and the second feeding electrode 10. The elements for fitting and pressing the measuring pad 6 are not shown in this Figures, too.

[0050] In the embodiment shown in Fig. 5 the outer measuring electrode 13 is divided into twelve segments. The cross-­section is taken along a line A-A crossing the fourth and tenth of the segments, signed by 13/4 and 13/10. The arrange­ment of the further electrodes is the same as shown in the Fig. 2.

[0051] A downhole measuring tool 1 in an embodiment with four measuring pads 6 is shown in cross section in Fig. 6, in a cross section taken along a line VI-VI determined according to Fig. 1. The four measuring pads 6/1, 6/2, 6/3, 6/4 are pressed by respective arms 7 to the investigated wall region 8 of the borehole and they bear the electrode systems applied to the measurements (not shown in this Figure). The arms 7 bearing the measuring pads 6/1, 6/2, 6/3 and 6/4 are immersed in the drilling mud 2 filling out the interior of the bore­hole. The mechanical construction of the arms 7 (not shown in detail either here or in connection with other Figures) ensures that the pads 6/1, 6/2, 6/3, 6/4 are pressed to the wall region 8 mad in the hard rock formation 5 by drilling the borehole in presence of the drilling mud 2. The pressing mechanism guarantees that during the measurements there is no direct electric contact between the drilling mud 2 and the electrode system facing the wall region 8 on the measuring pads 6/1, 6/2, 6/3, 6/4. The specific feature of the embodiment shown in Fig. 6 is that the electrode systems of the measuring pads 6/1, 6/2, 6/3 and 6/4 are fed with currents of different frequency values respectively f₁, f₂, f₃ and f₄ in order to avoid interference between the meas­urements carried out simultaneously by the measuring pads 6/1, 6/2, 6/3, 6/4. Obviously, the number of the measuring pads is not limited to four as shown in Fig. 6. Advanta­geously the frequency values are selected in the range to 1 kHz. It should be noted that the mechanical construc­tion of the arms 7 is per se well known and doesn't require more explanation.

[0052] A further very advantageous embodiment of the appa­ratus proposed by the present invention is shown in the form of a circuit diagram in Fig. 7. This circuit diagram is in most of the details similar to those represented in Figures 2 and 4. The measuring pad 6 belonging to a downhole measuring tool 1 (Fig. 1) is intended to determine the geometric orientation of the fractures reaching in a network system the wall region 8 of the borehole and to detect whether a mudcake or a mud layer is present on the wall of the borehole or between the measuring pad 6 and the wall of the borehole. In this arrangement the first feeding electrode for supplying the main current signed in the previous Figures by 9 is divided into more, e.g. twelve feeding segments, four of them signed by 9/1, 9/j, 9/o and 9/12. The segments of the first feeding electrode 9 are connected over respective first output measuring resistors ― four of them are shown signed by 18/1, 18/j, 18/p, 18/12 ― with an output of the main current generator 15. The circuit arrangment is generally the same as shown in Fig. 2.

[0053] The other output of the main current generator 15 is connected with the far return feeding electrode 24, the ring shaped first and second monitoring (detecting) elect­rodes 11 and 12 for measuring of potential values are connected with respective inputs of the controlled current generator 16 producing the bucking current, coupled through its outputs over the second output measuring resistor 17 to the second feeding electrode 10 and directly to the far return feeding electrode 24. The first output measuring electrodes including the resistors signed by 18/1, 18/j, 18/p, 18/12 are connected over their common points with the respective feeding segments, i.e. 9/1, 9/j, 9/p and 9/12 to respective inputs of ammeters, wherein Fig. 8 shows only four ammeters signed by 19/1, 19/j, 19/p, 19/12. The poles of the second output measuring resistor 17 are connected to to inputs of the auxiliary ammeter 20 for determining the value of the bucking current. The other poles of the first output measuring resistors 18/1, 18/j, 18/p, 18/12 etc. are united by a common conductor led to the respective other inputs of the ammteres 19/1, 19/j, 19/p, 19/12 etc. The ring shaped outer measuring electrode 13 and the generally point-­like central electrode 14 are coupled with the inputs of the unit 22 for measuring voltage difference. The central electrode 14 is connected with the voltmeter 23 for measur­ing the absolute value of the voltage of the central elect­rode 14 against the far reference electrode 25. The outputs of the ammeters 19/1, 19/j, 19/p, 19/12, auxiliary ammeter 20, unit 22 for measuring voltage difference and voltmeter 23 are coupled with third arithmetic data processing means 27 having outputs 27/1, 27/2, 27/3, 27/4, 27/5, 27/6, 27/7, 27/8 for forwarding respective digital signals according to the measured parameter values.

[0054] Complex arithmetic data processing means are repre­sented in Fig. 8. These means form an expanded version of the third arithmetic data processing means 27. The inputs of the complex arithmatic data processing means 28 are ge­nerally the outputs 27/1, 27/2, 27/3, 27/4, 27/5, 27/6, 27/7, 27/8 of the third arithmetic data processing means 27 and their outputs signed by 28/1, 28/2, 28/3, 28/4, 28/5, 28/6, 28/7, 28/8, 28/9 are independent on the state of the outputs 27/1, 27/2, 27/3, 27/4, 27/5, 27/6, 27/7 and 27/8 of the third arithmetic data processing means 27.

[0055] The embodiments of the apparatus proposed by the pre­ sent invention shown in Figures 1 to 8 are operated in the following way, whereby the essence of the method of the invention can be also exemplified.

[0056] The downhole investigating tool 1 illustrated in Fig. 1 is equipped with an inner housing which is electri­cally isolated from the drilling mud 2. This inner housing may include some or nearly all circuit units of the measur­ing system(s) offered by the invention. The arms 7 of the downgole investigasting tool 1 press the insulating carrier bodies 36 of the measuring pads 6 (signed also by 6/1, 6/2, 6/3 and 6/4) to the investigated wall region 8 of the bore­hole lowered in the hardrock formation 5. The measuring pads 6 bear the metallic electrodes which are necessary for generating the microelectric fields of the determination, for monitoring and detecting the generated electric fields, for regulating and measuring some parameters. The measuring pads 6 of the downhole investigating tool 1 are made of an insulating material, whereby the metallic electrodes are isolated each from other and the direct galvanic contact with the electrically conducting drilling mud is excluded. The metallic electrodes are contacted with the investigated wall region 8 of the hard rock formation 5 through a thin mud layer and if necessary this thin mud layer gives contact to the drilling mud 2.

[0057] The measurements are carried out by the downhole meas­uring tool 1 suspended in the borehole and forwarded therein in a substantially continuous movement. The well logging cable 3 contacts the downhole measuring tool 1 with an appropriate voltage supply unit (not shown in Figures), if necessary and it forms the way of forwarding the signals representing the measured values, the processed measurement data.

[0058] During the process of (continuous or discontinuous) lowering the downhole measuring tool 1 the feeding electrodes 9 and 10 generate at each measuring pad 6 microelectric fields automatically and continuously focused and controlled. The continuous generations of the microelectric field renders it possible to carry out the continous measurements relating to the fractures in the investigated wall region 8. The basis of processing the measured data is the sampling, wherein the frequency of sampling is selected to be as high as necessary for ensuring the possibility of producing a complex fracture picture by processing the measured data.

[0059] In the basic embodiment of the proposed invention the method realised ensures the detection of the fractures in the neighbourhood of the borehole with enhancement the pres­ence of the hydraulically conductive open fractures, forming parts of a network of communicating vessels and with sup­pression of the data assigned to the closed fractures which are the result of the process of lowering the borehole. This method is realised by the apparatus in its embodiment shown in Figures 2, 3 and 3A.

[0060] As it can be seen in Fig. 2, the main current gener­ator 15 supplies current (of intensity I₁) through the first output measuring resistor 18 and the ring shaped first feed­ing electrode 9, signed previously also by A₁, to the space of the measurements. The first feeding electrode 9 constitutes together with the first output measuring resistor 18 a se­ries member. The current supplied and fed into the hard rock formation comes back to the main current generator 15 over the far return feeding electrode 24 (B). The field generated by the current of intensity I₁ results in a control potential difference ΔU₁ measurable between the first and second ring shaped monitoring electrodes 11 and 12 (S₁ and S₂). The measured potential difference ΔU₁ is given to the control input of the controlled current generator 16 which is a high stability current generator for generating bucking current also with high speed of alteration, if necessary. The controlled current generator 16 produces the bucking current I₂ and feeds it into the measurement space over the second output measuring resistor 17 and the second feeding electrode 10 connected in series. The presence of the buck­ing current I₂ results in a countervoltage ΔU₂ ruling bet­ween the first and second monitoring electrodes 11 and 12, wherein the magnitude of the countervoltage ΔU₂ is equal to that of the potential difference ΔU₁ but it is of opposite sign. The resulted control potential between the first and second monitoring electrodes 11 and 12 is in this case of near zero value. In the practice, of course, the ideally zero value is not achievable, however, a very low level can be ensured. The low control potential value is the object of this regulation and it can be ensured in a continuous process always during the measurements. The microelectric field sharply focused for longer time according to the me­chanism depicted includes a region of minimal potential level constituting a ring shaped space part within the space limited by the first and second monitoring electrodes 11 and 12. This results in producing a potential barrier by the potential field of the bucking current I₂, the potential barrier preventing the flow of the measuring current in la­teral directions oblong the surface of the measuring pad 6 through the thin mud layer present between the measuring pad 6 and the wall region 8. This potential barrier prevents the lateral current flow in the case if the insulating ma­terial of the measuring pad 6 is pressed to a tight wall region. The flow of the bucking current I₂ is not excluded also in this case, because of lack of any potential barrier working against the second feeding electrode 10 assigned to the bucking current I₂. Hence, the bucking current I₂ can find the way of flowing through the thin mud layer to the basic mass of the drilling mud 2 characterized by re­latively high electric conductivity. It follows that in the case of investigating a tight wall region in the borehole the bucking current is very high in comparison to the main measuring current I₁, i.e. the output 21/2 of the first arithmetic data processing means forwards a fracture indica­tor I₁/I₂ of very low value, practically being equal to zero.

[0061] If the investigated wall region 8 of the borehole in­cludes a fracture terminating at the borehole and classified as an open fracture because of remarkable hydraulic conduc­tivity, this fracture traverses the line of the ring shaped feeding and monitoring electrodes 9, 10, 11, 12 and measur­ing electrode 13. This fracture of this kind comprises elect­ rically conductive liquid constituted generally by the drill­ing mud penetrating the fracture and in this conditions a current transport from the main current I₁ can be observed from the first ring shaped feeding electrode 9 to the space of the measurements and the intensity of the current flow is increased in dependency on the hydraulic conductivity of the open fracture, i.e. the ratio I₁/I₂ indicates that a fracture with increased hydraulic conductivity is present. This enhance­ment of the open fractures is the main object of the dimension­less current ration I₁/I₂ constituting a fractor indicator not only for open fractures.

[0062] This method does not give data referring to the clos­ed fractures which are not important. The closed fractures behave in this method in similar manner to the tight wall of the borehole, because they are not capable of giving a continuous way of flowing the main current I₁.

[0063] For carrying out the measurements the main ammeter 19 produces a signal with level proportional to the inten­sity of the main current I₁. The signal is forwarded to an input of the first arithmetic data processing means 21 from the first output measuring resistor 18. The auxiliary ammeter 20 forwards from the second output measuring re­sistor 17 also to the first arithmetic data processing means 21 a signal with level corresponding to the intensity of the bucking current I₂. The first arithmetic data pro­cessing means 21 generates the ratio I₁/I₂ and forwards it through the output 21/2 of the first arithmetic data processing means 21.

[0064] At the same time the unit 22 for measuring voltage dif­ference determines the potential difference U_MK between the ring shaped outer measuring electrode 13 (M) and the point-­like central electrode 14 (K) and forwards the corresponding signal to a respective input of the first arithmetic data processing means 21. The voltmeter 23 determines the absolute potential (voltage) U_K of the central electrode 14 taking as basis the potential level of the far reference electrode 25 applied even for voltage measurements (N). This measured value is also forwarded to the first arithmetic data pro­cessing means 21, which computes and generates on the out­put 21/1 a dimensionless second fracture indicator U_MK/U_K for differentiating the open fractures from the closed ones. It is to be noted that the second fracture indicator, i.e. the ratio U_MK/U_K has values remarkably low in the case of closed fractures and relatively high values if the fracture is open. Similarly to the first fracture indicator, i.e. to the ratio I₁/I₂ the second indicator has also val­ues increasing with the hydraulic conductivity (permitti­vity) of the open fractures.

[0065] The first arithmetic data processing means 21 computes further a normalized fracture indicator H, which is also a dimensionless parameter. The corresponding signal is for­warded by the output 21/3 of the first arithmetic data pro­cessing means 21. The normalized fracture indicator H is a modified product of the first and second fracture indicators and it can be given by the formula

wherein A and B are mathematical constants reflecting the given geophysical conditions. The modified product determined by the formula is capable of enhancing the open fractures from the background of the fractures and giving a signal level according to the hydraulic conductivity. The product has very low value in the case of the closed fractures. This results in a very clear picture of the fractures present in the region of the measurements. The normalized fracture indicator H reflects in a very sensitive manner the hydrau­lic conductivity of the open fractures and forms an ade­quate basis for analyzing the hydraulic conditions of the open fractures, a better basis than either the first frac­ture indicator I₁/I₂ or the second fracture indicator U_MK/U_K taken separately or together.

[0066] The method and apparatus depicted above has a further unexpected advantage, i.e. the absolute potential value U_K measured by the voltmeter 23 and the intensity I₁ of the main current measured by the ammeter 23 on the basis of the first output measuring resistor 18 give a resistance value which can be identified as apparent microresistivity R_a of the microelectric field generated by the main current. The first arithmetic data processing means 21 forwards on the output 21/4 a logarithmic signal according to the formula

The expression R_a = WU_K/I₁ with the constant W depending on the downhole measuring tool 1 is a conventional auxiliary parameter facilitating the investigations by reflecting the material of the hard rock formations and by improving the reliability of correlating the fracture indicators H, I₁/I₂ and U_MK/U_K of a given depth with other geophysic parameters determined to the same borehole lowered in the given hard rock formation.

[0067] By applying the modified circuit diagram shown in Fig. 4 and the pad according to the Figures 5 and 5A it is possible to determine the geometric configuration of the open fractures reaching the wall of the borehole and the difference of this configuration to the straight line arrangement, i.e. the measurements are capable of giving information on the fractures whether they tra­verse the hard rock formation along straight, dotted or curved lined.

[0068] A comparison of the Figure 4 with Figure 2 gives an evidence of similariity of the majority of the circuit elements shown in the Figures mentioned. The way of oper­ating the apparatus according to Figures 4, 5 and 5A is generally similar to that of the apparatus described above with reference to the Figures 2, 3 and 3A and it is pos­sible to carry out the same operations.

[0069] In the apparatus with circuit diagram represented by Figure 4 and with the pads 6 built up according to Figures 5 and 5A the outer measuring electrode 13 is segment­ed. The number of the segments can be as high as desired, it depends on the technical conditions. The practical re­sults show that the best is to apply from 4 to 24 segments, especially 12 segments - the 12 segment arrangement is shown in Figures 4 and 5.

[0070] The apparatus built up according to this embodiment carries out potential difference measurements for determin­ing the values U_MK,i assigned to the segments 13/1 to 13/n of the outer measuring electrode 13 (n means the number of the segments, in the embodiment shown n = 12, and i is the serial number of the segment). The potential difference U_MK,i assigned to the i-th segment of the outer measuring electrode 13 is measured by the unit 22/i - the Figure 4 shows only four of the segments signed by 13/1, 13/j, 13/m and 13/12 connected with measuring units 22/1, 22/j, 22/m and 22/12. The measuring units 22/i are all contacted with respective inputs of the second arithmetic data processing means 26.

[0071] The second arithmetic data processing means 26 car­ries out more operations than the first arithmetic data processing means 21, it includes more units, operational elements.

[0072] The second arithmetic data processing means 26 receives signals corresponding to the measured potential differences U_MK,i and generates in real time mode the mean value U_MK as part of the ratio U_MK/U_K, wherein U_K is the absolute potential of the central measuring electrode 14 related to the far reference electrode 25. The second arithmetic data processing means 26 produces the average fracture parameter expressed in the form of the following formula:

The average fracture parameter defined above as U_MK/U_K is a very good approximation of the fracture indicator U_MK/U_K defined above with reference to the embodiment of the apparatus according to the invention shown in Figures 2, 3 and 3A, i.e. the embodiments defined above are cap­able of ensuring the same parameters. The average fracture parameter is forwarded further by the output 26/1 of the second arithmetic data processing means 26. Of course the mentioned means are capable of generating the second fracture indicator I₁/I₂ defined above (forwarded by the output 26/2), and the normalized fracture indicator in modified form:

(forwarded by the output 26/3) and the logarithmic apparent microresistivity log R_a = log (WU_K/I₁) (forwarded by the output 26/4), as well. The mentioned parameters are computed and forwarded in real time mode, they can be stored and applied for creating a full picture of the borehole environ­ment, if they are assigned to different depth levels in the borehole.

[0073] For determining the geometric arrangement of the open fractures the second arithmetic data processing means 26 produces in real time mode and stores the elementary fracture indicators U_MK,i/U_K forming ratios of the potential differences measured for the i-th segment and of the absolute potential U_K measured by the central electrode 14. These elementary fracture indicators are forwarded by the output 26/5. The second arithmetic data processing means 26 selects in real time or off-line mode the two maximal values from the measured elementary indicators, if they are not assigned to two adjacent segments. In the example of Figure 4 the segments so selected are 13/j and 13/m. The maximal elementa­ry fracture indicators U_MK,j/U_K and U_MK,m/U_K are forwarded on the outputs 26/6 and 26/7 of the second arithmetic data processing means 26. The selected values are also stored together with data necessary for identifying the place of determining them.

[0074] The series of the elementary fracture indicators U_MK,i/U_K measured versus depth of the borehole and the selected local maximum value U_MK,j/U_K and U_MK,m/U_K form an adequate basis for determining the geometric network of the open fractures reaching the wall of the borehole including the determination of the directions of the frac­tures - they can stretch along broken and curved lines, exceptionally along straight lines. The determination of the geometry of the fracture network is generally carried out in off line mode, after completing the measurements and collecting all necessary data and parameters. This process of determining is a normal object for a computer system.

[0075] Turning now to Fig. 6 it can be seen, how more meas­urements can be completed simultaneously oblong the circum­ference of the borehole. The Figure shows a four pad meas­uring arrangement. The focused microelectric field is in this case generated in four different places and the simul­taneous measurements must not interfere one with another if the reliability of the measured parameter should be ensured.

[0076] According to the invention the proposed method of simultaneous measurements is carried out by applying four different frequency values signed by f₁, f₂, f₃ and f₄. The four frequency values assigned to the four pads ar­ranged at the wall of the borehole should differ one from the other with a frequency distance which is as high as necessary for avoiding the interference. Let the frequency increase with the indices: f₁ < f₂ < f₃ < f₄. They are assigned to the pads as shown in Fig. 6. The frequency difference f₂ - f₄ between the neighbouring pads 6/1 and 6/2, f₄ - f₁ between the adjacent pads 6/2 and 6/3 and f₁ - f₃ between the neighbouring pads 6/3 and 6/4 can be selected to be relatively big in order to ensure the independency of the microelectric fields generated in a controlled manner for carrying out the measurements. It seems to be an interference problem between the neighbouring pads 6/4 and 6/1 because of applying the adjacent frequency values f₂ and f₃. Of course, by selecting a relatively big difference between f₂ and f₃ the problem can be avoided and no interference problem occurs.

[0077] The more pad system applied according to Figure 6 is not specific to the construction of the measuring pads 6. There is no difference, whether the measuring pads 6/1, 6/2, 6/3 and 6/4 are constructed according to Figure 2, 4 or 7, with electrode systems according to Figures 3, 3A, 5, 5A or 7. In all cases the measured data are processed by the corresponding arithmetic data processing means in real time mode and stored versus depth defined along the axis of the borehole. During the measurements the real time display of the measured data is generally realised only in connection with one of the pads 6, e.g. the pad 6/1, capable of determining the fracture indicators (I₁/I₂)₁,

and (H)₁, (H′)₁, further the apparent microresistivity values expressed in logarithmic forms:

The mentioned values can displayed during the measurements and they illustrate the conditions ruling in the borehole.

[0078] The parameters measured on the different pads in a real time mode are stored and in an off-line arrangement they can be applied for determining the image of the network of the open fractures reaching the circumference of the borehole. Of course, the measured data can be applied in an an obvious way for displaying the parameters related to only one of the cross-sections of the borehole.

[0079] The determination of the continuous picture of the fracture network can be carried out by applying a specific software forming no part of the present invention.

[0080] In a further embodiment of the present invention the geometry of the open fractures reaching the wall of the borehole is determined by the measurement of current intensities I_1,i (i means an integer from 1 to n) charac­terizing the segments of the first feeding electrode 9 divided into n segments. In this embodiment it is possible to determine the thickness of the mud layer present be­tween the pad of the downhole measuring tool 1 and the wall of the borehole (rugosity) or the thickness of the mudcake, to differentiate the mudcake of uniform thick­ness from the rugosity of uneven thickness. This embodi­ment of the proposed method is carried out by the appa­ratus with circuit diagram shown in Figure 7 having, if necessary an expanded complex arithmetic data processing means 28 according to Figure 8.

[0081] Turning now to Figures 2, 4 and 7 it is clear that the most of the elements are common and they do not require further explanation which repeats only the aforesaid. In the case of the apparatus built up according to the Figure 7 the elements common with the apparatus according to Figure 2 or 4 are operated in the samy way.

[0082] In the apparatus of the invention realised with the circuit diagram shown in Figure 7 the main difference in comparison with the embodiments previously described lies in the application of the segmented first feeding electrode 9 for supllying in the main current. The number of the segments applied is not specific, it depends on the given conditions. According to the practice the most advisable is to divide the first feeding electrode 9 into 4 to 24 segments, preferably into 12 segments as it is shown in Fig. 7. The apparatus determines the current intensities I_1,i for each of the segments 9/i, wherein i = 1 to n, and n means the number of the segments. In this measurement the first output measuring resistors 18/i (18/1, 18/j, 18/p, 18/12) are applied. One of the poles of the first output measuring resistors 18/i is contacted through a common conductor to the main current generator 15 - this ensures a common potential for each segment 9/i (9/1, 9/j, 9/p, 9/12) of the first feeding electrode (ring A₁). The ammeters 19/i (19/1, 19/j, 19/p, 19/12 measure the current intensities in the segments of the ring A₁ and the segment current intensities I_1,i are forwarded to the required input terminals of the third arithmetic data processing means 27. The further input terminals of the third arithmetic data processing means 27 receive the following data: intensity I₂ of the bucking current measured through the low resistivity second output measuring resistor 17, the voltage difference U_MK measured by the unit 22 between the first and second monitoring electrodes 13 and 14, the absolute potential value U_K meas­ured between the central electrode 14 and the far reference electrode 25 by the voltmeter 23.

[0083] The parameters measured and forwarded to respective inputs of the third arithmetic data processing means 27 are in a real time process transformed and the following dimensionless ratios are determined and transmitted through the respective outputs: fracture indicators U_MK/U_K, I_1,1/I₂, ..., I_1,j/I₂, ..., I_1,p/I₂, ..., I_1,12/I₂ together with the serial numbers of the segments of the first feeding electrode, the sum

of segment current intensities, the modified fracture indicator H˝ formed on the basis of the first fracture indi­cator and the sum mentioned according to the formula

which determines the hydraulic conductivity of the frac­tures, and the logarithmic apparent resistivity expressed by the formula

The mentioned parameter are measured and computed in a con­tinuous process during lowering the downhole measuring tool 1. They are also stored immediately after computing. The first fracture indicator U_MK/U_K, the mentioned sum of the segment current intensities, the modified fracture indicator H˝ can be registered also versus depth in selected cross sections of the borehole, wherein the momentary position of the downhole measuring tool 1 is registered, too.

[0084] In Figure 8 a very schematic representation of a complex arithmetic data processing means 28 is shown in contact with the outputs 27/1 to 27/8 of the third arithme­tic data processing means 27. The complex means 28 receives the output signals of the third arithmetic data processing means and forwards on the output 28/1 a signal corresponding to the normalized average value of the segment current in­tensities according to the formula

on the output 28/2 the normalized difference of the segment current intensities from the average value according to the formula

for each of the segments, on the outputs 28/3, 28/4, 28/5 and 28/6 those of the mentioned normalized differences (at most four with serial number e.g. j, m, p and v) which exceed a predetermined threshold value ε together with the corresponding serial numbers of the segments, i.e. the values

wherein w = j, m, p, v. The outputs 28/3, 28/4, 28/5 and 28/6 forward signals for determining the direction and the network geometry of the fractures. A further fracture indicator of improved informative content signed previously by H_e is generated and forwarded through the output 28/7 according to the formula

for determining the hydraulic conductivity of the fractures. If the comparison results in the consequence that there is no fracture indicator exceeding the threshold value ε, than the output 28/8 is applied for signalizing the lack of any fractures on the place of the investigation. This output is applied also for forwarding the thickness t_m of the rugose mud layer present between the insulating pad 6 and the tight wall of the borehole or of the mudcake present in the borehole. The thickness t_m can be determined on the basis of a functional dependency between the thick­ness t_m and the ratio I_1,i/I₂ and this is also an object of the complex arithmetic data processing means 28. If the output 28/8 is active, the complex arithmetic data pro­cessing means 28 is applied also for testing whether the normalizing current differences (I_1,i - I_1,i)/I₂ forwarded on the output 28/2 for each segment (i = 1 to n) exceed a predetermined second threshold value ω or not. If all normalized current differences are greater than this second threshold value ω, than output 28/9 signalizes the presence of a mud layer between the rugose wall of the borehole and the insulating measuring pad 6; in opposite case, if the normalized current differences all are smaller than the sec­ond threshold value ω, than the output 28/9 is applied for signalizing that on the wall of the borehole there is a mudcake on the place investigated. The complex arithmetic data processing means 28 operates in off-line mode and include storage means for receiving the data from the out­puts. The output data are applied for creating a display about the geometric arrangement of the fractures in the network, the geometric arrangement is determined generally versus depth along the axis of the borehole. The modified fracture indicator H_e (output 28/7), the thickness t_m of the mud layer or mudcake (output 28/8) and the specific characteristic data forwarded by the output 28/9 are gene­rally illustrated in function of the depth, depending on the position of the downhole measuring tool 1.

[0085] The method according to the invention realised by the novel arrangements shown in Figures 1 to 8 is capable of carrying out the following:

1. measuring, indicating the hydraulically conduc­tive open fractures and differentiating them from the nonconductive closed fractures;

2. measuring and determining the geometric arrange­ment of the open, hydraulically conductive fractures by supposing their broken and curved line shapes;

3. measuring and determining - on the hard rock wall regions without open fractures - the thickness of a rugose mud layer or a mudcake precipitated on the wall of the bore­hole, further differentiating the regions covered by the mudcake from the regions with uneven, rugose wall surface formations, the rugose mud layer being present between the insulating material of the measuring pad and the wall of the borehole;

4. measuring and determining fracture indicators H, H′, H˝ or M and the fracture indicators H_e and M_e of improved informative content, whereby the hydraulic con­ductivity of the open fractures can be measured on the places of the investigation;

5. applying in a multiplicative way the features mentioned above, i.e. carrying out simultaneous measure­ments in different points of the borehole and obtaining thereby a more complex image of the open fractures present at the wall of the borehole.

Claims

1. Method for carrying out measurements on open and closed fractures in a hard rock formation pierced by a bore­hole, comprising the steps of pressing in a downhole measuring tool at least one measuring pad made of insulating material to a region to be investigated in a borehole lowered in a hard rock formation, the pad including metallic electrodes to be contacted with the region of investigation, generating by means of a first group of the electrodes a controlled mic­roelectric field penetrating the hard rock formation, measur­ing by means of a second group of the electrodes current and voltage conditions created by the first group of the elect­rodes and identifying on the basis of the measured conditions hydraulically conducting open fractures and closed fractures of lowe hydraulic conductivity present in the hard rock for­mation either because of lowering the borehole or in the form of openings filled with concrete rock material diminish­ing the hydraulic conductivity,
characterized in the steps of supplying by means of a ring shaped first feeding electrode arranged concentrically aroung a point-like central electrode a main current into the region investigated, supplying by means of a ring shaped second feeding electrode arranged around the first feeding electrode a bucking current into the region investigated, measuring in two monitoring electrodes arranged between the first and second feeding electrodes a control potential, regulating the bucking current in order to ensure a value as low as possible for the control potential measured between the monitoring electrodes, measuring the main current, the controlled bucking current, the potential difference between the central electrode and an outer meas­uring electrode surrounding the central electrode and the absolute value of the potential of the central electrode, the central electrode, outer measuring electrode, first and second feeding electrodes and first and second monitoring electrodes forming a substantially concentric electrode system built up in the insulating material of the measuring pad, creating dimensionless ratios of the currents and of the measured potentials and identifying and differentiating the open and closed fractures on the basis of the dimension­less ratios measured in a substantially continuous process during movement of the at least one measuring pad of the downhole measuring tool along the axis of the borehole.

2. The method as set forth in claim 1, characte­rized in the step of creating the dimensionless ratios I₁/I₂, U_MK/U_K and a normalized product thereof in the form of

wherein I₁ means the main current, I₂ the bucking current, U_MK the potential difference between the central electrode and the outer measuring electrode, U_K is the absolute poten­tial in the place occupied by the central electrode and A and B are mathematical constants with values following from the given geophysic conditions of the hard rock for­mation.

3. The method as set forth in claim 1 or 2, char­acterized in comprising the further steps of dividing the outer measuring electrode into at least 4, and at most into 24, advantageously into 12 segments, measuring the potential difference U_MK,i with respect to the central electrode for each segment, wherein i means the serial number of the segments with integer values i = 1 to n, creating dimensionless ratios U_MK,i/U_K, where U_K means the absolute potential of the central electrode, selecting the two maximal values from the dimensionless ratios U_MK,i/U_K with serial numbers i differing by more than one, carrying out determination of the dimensionless ratios in a substan­tially continuous way along the axis of the borehole by translating the at least one measuring pad in the borehole and determining the direction of traversing the surface of the pad by the open fracture on the basis of more meas­urements.

4. The method as set forth in claim 3, charac­terized in computing an average value

on the basis of the dimensionless ratios U_MK,i/U_K, creating the dimensionless ratio I₁/I₂ and a normalized product H′ of the average value and the dimensionless current ratio according to the formula

for differentiating the closed fractures from the open fractures.

5. The method as set forth in any of claims 1 to 4, characterized in the further steps of carrying out simultaneous measurements in at least 2, at most 8, advantageously in 4 regions determined along the circum­ference of the borehole, applying during the measurements features preventing interference between the results of the simultaneous measurements and creating a full image of the fracture network in the wall of the borehole along the axis of the borehole.

6. The method as set forth in claim 5, charac­terized in the step of carrying out the simultaneous measurements by means of measuring pads connected with supply units forwarding measuring currents of different frequency values for preventing interference.

7. The method as set forth in any of claims 1 to 6, characterized in the further steps of dividing the first feeding electrode into at least 4, at most 24, advantageously into 12 segments, connecting the segments to a common potential, measuring the elementary currents I_1,i of the segments, wherein i means the serial number of the segments in the range i = 1 to n, comparing the elementary currents I_1,i to the bucking current I₂ and creating an elementary fracture indicator I_1,i/I₂ for each of the segments for determining the direction of a current transport through a mudcake present between the measuring pad and the wall region to be investigated in the borehole, selecting at most four to the elementary fracture indicators belonging to the segments, the four elementary fracture indicators with maximal values and having sering numbers j, m, p, v, the segments being se­parated one from another by at least one of the segments, determining the directions between the segments with serial numbers j and p, further m and v, the angle position of the lines connecting the segments with serial numbers j, p, m and v, translating the central electrode along the wall of the borehole and analysing the results of measurements carried out continuously.

8. The method as set forth in claim 7, charac­terized in the further steps of determining the mean value I_1,i of the elementary fracture indicators I_1,i, then computing the difference ratios (I_1,i - I_1,i)/I₂ for each segment and selecting from the difference ratios at most four exceeding a predetermined threshold ε and having serial numbers j, m, p, v, creating a modified frac­ture indicator H_e according to the formula

and determining the hydraulic conductivity of the open fractures on the basis of the modified fracture indicator H_e at the place of the investigation.

9. The method as set forth in claim 7 or 8, charac­terized in the further steps of checking the computed difference ratios (I_1,i - I_1,i)/I₂ for each segments and detecting presence of an uneven mud layer or mudcake between the measuring pad and the wall of the borehole on the basis of the number of the computed difference ratios remaining under the predetermined threshold ε and defining thickness of the mudcake or uneven mud layer on the basis of the value I_1,i/I₂.

10. Apparatus for carrying out measurements on open and closed fractures in a hard rock formation pierced by a borehole, comprising at least one measuring pad arrang­ed on an arm of a downhole measuring tool suspended in the borehole, the measuring pad made of insulating mate­rial and comprising an electrode system, and a current and potential measurement system connected with data pro­cessing means, characterized in the electrode system including a point-like central electrode (14) and an outer measuring electrode (13) forming a first group of electrodes for current measurement, the first group surrounded by a first feeding electrode (9) and a second feeding electrode (10) limiting a first and a second monitor­ing electrodes (11, 12) for regulating bucking current (I₂) supplied by the second feeding electrode (10), the first and second monitoring electrodes (11, 12) and the first and second feeding electrodes (9, 10) forming a second group of electrodes, wherein the first and second monitor­ing electrodes (11, 12) are connected to respective inputs of a controlled current generator (16) for supplying the bucking current (I₂) to the second feeding electrode (10), the first feeding electrode (9) is connected to a main current generator (15).

11. The apparatus as set forth in claim 10, char­acterized in that the outer measuring electrode (13) is divided into ring segments (13/i) and the segments (13/i) are connected by respective voltametric units (22/i) to an arithmetic data processing means (26).

12. The apparatus as set forth in claim 10 or 11, characterized in that the first feeding electrode (9) is divided into segments (9/i) connected over respective output measuring resistors (18/i) to the main current gene­rator (15) and ammeters (19/i) for measuring the main cur­rent, wherein the segments (9/i) are connected to a common potential, the ammeters (19/i) are connected with an arith­metic data processing means (27).

13. The apparatus as set forth in claim 11 or 12, characterized in that the number of segments (9/i, 13/i) is at least four, at most twenty four, advanta­geously twelve.

14. The apparatus as set forth in any of claims 10 to 13, characterized in applying means for supplying currents of different frequency values to different measuring pads, wherein the number of the measuring pads is at least two, at most eight, advantageously four.

Drawing